This invention relates to an apparatus for effecting the continuous separation of particulate solids from a liquid suspension of said solids. In a particular embodiment it relates to a clarifier, although the concepts embodied in the apparatus could be applied to filtration or drying units.
The embodiment of the invention which has been built and tested was a clarifier. This clarifier was designed to treat the clay-in-water tailings suspension which is a waste stream from a facility which extracts bitumen from mined tar sands using a process known as the hot water process. While not limited to such use, the clarifier will be discussed below in connection with certain needs and requirements which arise when one comes to treating such tailings. Also, the specific form of clarifier which has been built and tested was one which incorporated electrodes and used the process mechanisms of electrophoresis and reverse osmosis to collect and dry the solids present in such tailings.
In northern Alberta, there are presently two commercial facilities or plants which extract bitumen from mined tar sands using the hot water extraction process. These are very large plants--they each produce about 100,000-150,000 barrels of partly refined oil product per day.
The tar sand itself is something akin to a dark fine beach sand. It comprises relatively coarse quartz grains sheathed in thin films or envelopes of connate water. The bitumen or oil is entrapped in the interstices between the water-sheathed sand grains. Minute clay particles, referred to as "fines", are contained in the water sheaths.
In accordance with the first step of the hot water process, the tar sand is mixed with hot water and a small amount of "process aid" (usually NaOH), and mulled in a rotating drum. This operation is referred to as "conditioning" the tar sand. It effects a preliminary dispersion of the bitumen and solids. The resultant slurry is then diluted with additional hot water and introduced into a thickener-like vessel known as a primary separation vessel. The diluted slurry is held in this vessel under quiescent conditions for a period of time. The largest part of the bitumen becomes associated with air bubbles, entrained during conditioning, and rises to form a froth surface layer. This froth is recovered. Most of the heavy sand particles, together with water, some fines, some bitumen, some process aid, and surfactants produced by reaction of the process aid and bitumen, sink and leave the vessel through a bottom outlet. This stream is referred to as "primary tailings". A large portion of the fines and some non-buoyant bitumen collect in the mid-section of the vessel contents A dragstream from this zone, termed "middlings", is withdrawn and introduced into a series of induced air flotation cells. Here the middlings are contacted with a flood of minute air bubbles and much of the contained bitumen is aerated and rises to form "secondary froth", which is recovered. A tailings product, referred to as "secondary tailings", is passed out of the bottom outlets of the flotation cells. These secondary tailings comprise water, some fines, some bitumen, some process aid, and some surfactants.
The primary and secondary tailings are combined and discharged onto the shore of a large tailings pond. Here, the heavy sand grains immediately settle out and form a beach.
This leaves a suspension of fines in water, bitumen, process aid, and surfactants. The fines settle, but not into the form of a compact mass. Instead they form a structure somewhat like a house of cards. This bottom portion of the pond is a mixture known to the industry as "sludge" and it comprises fines, water, some sand, some bitumen, and small quantities of NaOH and surfactants. The mineral content ranges from 11 to 40% by weight.
The volume of sludge so formed is enormous. In the order of 1 gallon/minute is formed for every 10 barrels/day of oil produced. It accumulates at a rate in the order of 20,000 acre feet per year per plant. It has been calculated that the tailings pond for one of these plants will cover 10 square miles at the end of a 25 year period of operation.
There is a substantial quantity of process water tied up in the sludge. It cannot be recycled to the hot water process unless its solids content is reduced to about 3% by weight or less. If the solids were to be separated from the water, it would be desirable that the moisture in the solids fraction be in the order of 50% by weight or less, so that the solids could be worked by bulldozers and the like.
From the foregoing, it will be noted that a clarifier for this particular sludge feedstock should be characterized by the following:
it should be continuous and capable of treating the large volume at low cost; and
it should be capable of separating the solids and liquid to produce a clarified water product containing only up to 3% solids and a dried solids product containing in the order of 50% moisture of less.
Applicant has worked for several years developing a viable clarifier for this purpose.
The approach taken was to use spaced anode and cathode electrodes, to cause deposition of solids on the anode by electrophoresis and to dry the deposit by driving water out of it by reverse osmosis. As is known, if an anode and cathode are submerged in spaced apart relation in a solids-water suspension, and potential is applied to the electrodes, the solids will migrate to and collect on the anode by electrophoresis and, with sufficient retention time, water trapped in the solids deposit will move out of it by reverse osmosis.
Applicant evolved a system comprising:
a trough for containing the sludge to be treated;
a travelling anode comprising anode bars transversely mounted on the surface of an endless, driven conveyor belt disposed at an angle and partly submerged in the suspension, so that, on the upper leg of travel, the anode bars moved from a low starting point in the suspension, up through the suspension, and then out of the suspension and beyond the end of the trough, before rounding a roll and commencing a downwardly inclined leg of travel back to the starting point (the solids dropping off at the roll);
and an elongate, fixed cathode suspended in the suspension in parallel spaced relation to the upper leg of the anode.
In the operation of this unit, it was found that, if the electrodes were closely spaced, in the order of 2-5", if a voltage of 4-25 volts was used, if the sludge was suitably chemically pre-conditioned with 0.5 to 3.5 grams/liter of lime, if the pH was maintained in the range 8.5 to 11.5 using a carbonate-forming reagent such as CO.sub.2, and if a retention time in the order of 4 hours was observed, then the desired objectives, of obtaining a desirably dry solids deposit and a desirably clarified water, could be achieved.
However, testing and cost analysis established a fundamental shortcoming of this system, namely: that because the volume of sludge to be treated is so large, and the retention time is so long, and the depth of sludge which can be treated between one pair of electrodes is so small, one would require a very large surface area of electrodes to carry out the treatment. This would result in an inordinate extent of land use and capital costs that would be impractical.
So it was with this background that applicant set out to device a clarifier capable of providing a large active surface area relative to its overall size and land use, which clarifier would incorporate gravity settling and which would be capable of transporting the solids out of and away from the liquid, for disposal.